MD2/MD3 support rewritten + optimizations

Backported from Roly Poly Putt, this is my code for a common model format. I also included two loaders, for both MD2 and MD3. This abstracts the rendering code away from the idiosyncrasies of a particular model format, making it a lot easier to add support for others (I also have a non-animated MS3D loader that I may contribute in the future).

This also includes some optimization -- it should now be possible to push a lot more polygons in the renderer itself.